Well, I'm back again. I think I've messed up my CSE - directly from the system reset, it has an error code 0x13 (error reading, programming, or erasing one of the CSE flash blocks). I decided to try and follow the instructions for resetting its flash space, but these involve working through the CSE itself, and any CSE commands I issue are ignored and it throws a 0x02 error code (command sequence error). No combination of CANCEL and INIT_CSE commands worked. CANCEL is the only command the CSE will accept without throwing that 0x02 error.
I've tried looking through AN4234, but a lot of details seem to be outdated for the MPC5777C.
Any thoughts as to what might be wrong?